It is known in the art of data communications that transmission of data encoded in a nonreturn to zero (NRZ) format forms an asymmetrical waveform around the zero reference generating a bias on the transmission line. The bias varies as a function of the data pattern causing the zero crossing points to fluctuate. Fluctuations in the zero crossing points are reflected as data jitter in a remotely located receiver. It is also known that coding data in a biphase format, such as the Manchester code, provides a balanced transmitted waveform which does not contain a net DC bias. The so-called biphase code provides for a transition in each baud period so that the transmitted signal is balanced. In other words, the time averaged value on the transmission line is zero. The use of such a biphase code has problems associated with it as well. Conventional transmission lines, such as a twisted-pair cable, exhibit a lossy or low pass frequency impedance which intensifies with increasing cable length. In accordance with the biphase encoding format, adjacent data symbols having different logic values produce a waveform which shifts between two frequencies (f and f/2) so as to achieve a transition in every baud period. Shifting between frequencies produces a so-called fat bit for different adjacent data symbols and a so-called skinny bit for like adjacent data symbols. The frequency dependent impedance of the transmission line disparately distorts the skinny bits in comparison with the fat bits. Alternatively expressed, the amplitude and phase of the waveform at frequency f is distorted more severely than that at a frequency f/2. The unequal treatment causes adjacent symbols to spread into each other forming distortion as intersymbol interference. Predistortion, commonly called transmit equalization, ameliorates the dissimilar effects of the transmission line by lopping off a portion of the fat bit so as to reduce its relative power content to that of a skinny bit.
In general, another concern with transmitting digital data is the high frequency composition of the sharp transitional edges. Before transmission, the waveform is typically passed through a low pass transmit filter. The transmit filter rounds off the sharp edges to suppress electromagnetic radiation and limits the frequency spectrum so as to minimize intersymbol interference. The properties of the transmit filter are selected such that zero crossing points of adjacent data symbols at bauds T-1 and T+1 coincide in time with the sample point of a data symbol at baud T. In this manner, unwanted contributions from adjacent symbols are minimized, substantially reducing the intersymbol interference.
Referring now to FIGS. 1A and 1B, are simplified block diagram illustrating the prior art approach to data transmission utilizing equalization by predistortion. NRZ data is received by an attachment unit interface (AUI) transmit receiver 10 in a communication network. Among other functions, the AUI transmit receiver 10 conditions and level shifts the received signals to an appropriate logic level. Control and encoding logic 12 compares adjacent data symbols for like logic levels and converts the data from NRZ to a Manchester encoding format. Data comparison outputs of logic circuitry 12 selectively energize buffers 14-20. Buffers 14 and 18 are coupled through a resistor having a low ohmic value (R) and through a transmit filter 22 to a primary winding of a transformer 24. Buffers 16 and 20 are wired ORed with buffers 14 and 18, respectively, through a resistor having a high ohmic value, typically on the order of ten times R. When energized, buffers 14 and 18 impress a fullstep voltage substantially spanning the supply rails across the primary winding of transformer 24. Buffers 16 and 20 when energized, impress a halfstep voltage across the primary winding of transformer 24 having a span between the negative rail and an intermediate value between the rails. Buffers 14 and 20 are selectively energized when logic circuitry 12 detects two adjacent data symbols of the same value. Likewise, buffers 16 and 20 are energized when logic circuitry 12 detects two adjacent data symbols having different values. A staircase-like waveform results across nodes 26-26' when a stream of data containing like and different data symbols is detected by logic circuitry 12.
Reference is now made to FIG. 2 for an illustration of the relationships between the various waveforms. Data in NRZ format 28 is received by the AUI transmit receiver 10. Logic circuitry 12 internally converts the data into a Manchester format 30. Logic circuitry 12 selectively energizes buffers 14-20 to impress the staircase waveform 32 across nodes 26-26'. Transmit filter 22, typically a high order low pass filter having a high Q, smooths out the staircase waveform 32 to a limited frequency spectrum waveform 34. To achieve a high order, high Q filter at the frequencies of interest, transmit filter 22 must be realized with passive elements. The passive embodiment is space consuming, subject to wide variances in component values and does not lend itself to effective consolidation onto an integrated circuit.
In accordance with the principles of the present invention, a system and method for transmit equalization is disclosed without the need for an external filter or external filter components. The invention reduces harmonic distortion and improves zero crossing jitter without space consuming, wide tolerance passive components and is completely realized in silicon circuitry. The present invention provides transmit equalization and suppression of electromagnetic radiation utilizing precoded waveforms in memory. Predetermined waveforms reduces system complexity, decreases system cost and increases system reliability.